Solvent effects square planar substitutions

While the trans effect is the major factor governing square-planar substitution reactions other factors contribute. These factors are the electronic effect of cis ligands (generally relatively unimportant), leaving-group effects, effect of the entering ligand, and solvent effects. 

Pearson and Sweigart have reported one of the first kinetic studies involving substitution at square-planar nickel(n), in Ni"-dithiolate complexes. They discuss nucleophilic reactivity, leaving group, and trans effects, and report that the reactions follow an associative pathway, as expected. The solvent path is not important and, although for Pt" the formation of a five-co-ordinate intermediate may be rate-determining, for Ni" this step is fast compared with subsequent steps. The formation constant of the five-co-ordinate intermediate does, however, markedly affect the overall rates, and it seems that the trans effect for nickel(ii) operates through the stability of this intermediate. 

Inorganic kineticists with an interest in solvent medium effects have not been generously supplied with reviews recently. Solvent paths and solvento-inter-mediates in substitution at square-planar centres have been discussed in a well referenced review. Unfortunately, another fully referenced and undoubtedly important review, on solvent properties of significance in determining reactivity in substitution at nickel(ii), has appeared in a periodical access to which may prove difficult for many readers. The contents of another, more general, review of solvent effects on kinetics, which includes discussion of appropriate physical models, may prove equally elusive. ... 

Solvent The identification of solvent path for the substitution reactions in square planar complexes has increased the importance of solvent effect on the rate. The solvents with greater coordinating abfiity show faster rates of substitution reactions. The observed rate constants for some of the popular solvents vary as ROH < H2O < CH3NO2 < (CH3)2S0... 

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